This invention relates to implantable articles and methods for manufacturing such articles. More particularly, the invention relates to bone prostheses and processes for their manufacture.
There are known to exist many designs for and methods of manufacturing implantable bone prostheses. Bone prostheses include components of artificial joints, such as elbows, hips, knees, and shoulders. An important consideration in the design and manufacture of implantable prostheses of this type is that the prosthesis have adequate fixation when implanted within the body, be structurally strong, and in articulation areas, have hard, long lasting wear surfaces.
Early designs of implantable articles relied upon the use of cements, such as polymethylmethacrylate to anchor the implant to surrounding bone, or to anchor pieces of the implant to each other. The use of such cements can have some advantages, such as enhancing the range of fit while providing an immediate and secure fixation that does not develop free play, or lead to erosion of the joining bone faces post operatively. However, the current trend is to use these cements to a lesser extent because of their tendency to lose adhesive properties over time and the possibility that the cements contribute to wear debris within a joint. Recently, implantable bone prostheses have been designed such that they encourage the growth of trabecular bone tissue around the implant, principally by arranging that the surface of the implantable bone prosthesis is irregular or textured in a defined way. The promotion of newly-formed hard tissue in and around the textured surface of the prosthesis has been found to provide good fixation of the prosthesis within the body. A greater degree of bone fixation can usually be achieved where bone contacting surfaces of an implantable prosthesis are more porous or irregular.
Such porous or irregular surfaces can be provided in implantable articles by a variety of techniques. In some instances, an irregular surface pattern or surface porosity is formed by embossing, chemical etching, milling or other machining operations, or plasma spray metallization. Textured surfaces are also applied by joining one or more separate surface plate inserts to an exterior surface of the prosthesis to provide separate porous or pore-forming surfaces. Separate pore-forming surfaces can be joined to or formed on an implantable bone prosthesis by sintering metal particles or powder to a surface of the prosthesis, or textured surfaces may be formed by soldering wire-based pads or grids to provide the desired relief surface features.
Similarly, wear surfaces of an articulation component are required to be smooth and hard, and a number of techniques have been developed to arrange that those regions of a bone prosthesis have appropriate surface properties. Ceramic material has many desirable properties for this application, since it is extremely hard, may be highly polished and is biologically inert. Unfortunately, a bone prosthesis made entirely of ceramic is subject to breakage, while ceramic coatings selectively placed on a metal prosthesis may be subject to cracking, chipping or spalling. If one attempts to overcome these problems by forming a set of ceramic portions of a prosthesis as blocks that are fitted into machined bores or dovetails of the metal prosthesis, one is limited to rather simple constructions, dictated by the available machine shapes.
As an intermediate approach, various surface hardening techniques have been proposed for metal prostheses, such as carbide or nitride surface treatments of the metal. Ion beam bombardment has also been proposed to enhance the depth of such surface transformation. These approaches, however, while providing greater toughness and fracture resistance than fired-on or cemented ceramic surface elements, affect only a very thin surface layer, typically under several micrometers deep. They also involve relatively costly and complex equipment and fabrication processes.
For production runs, implantable articles such as bone prostheses are preferably made by an investment casting process. In general, investment casting first requires the making of a solid model of the article to be cast, the model being made from a meltable casting wax through a molding operation such as injection molding. Once the solid model is made, one or more of the solid models are fixed to a wax tree, which is then encased, along with the attached models, in a refractory binder material to form runners interconnecting mold cavities filled with wax. After repeatedly dipping the assembly in a ceramic slurry coating and drying the coating between dips to form a wax-filled green shell, which is dried, the shell is heated to a temperature sufficient to melt and extract the casting wax from within. Thereafter, the shell may be sintered or fired at an even higher temperature, to form a refractory ceramic mold. High temperature heating also burns off any residues. Molten metal is then poured into the investment assembly to entirely fill the cavities once occupied by the wax and to form cast articles having the shape of the hollows left by the wax models. This process may be used to make strong metal bone prostheses, which may subsequently have their bearing surfaces treated or covered to form hardened regions, in the manner discussed above.
As is apparent from the foregoing discussion, the processes for making an implantable prosthesis currently involve many distinct steps in which, for example, different operations of casting, machining, heating and surface treatment or coating may be required in order for a single component to possess a body with acceptable strength and a surface with acceptable wear characteristics. Accordingly, there is a need for bone prostheses having improved load bearing and surface wear characteristics. There is also a need for improved methods of manufacturing prostheses having such characteristics.
It is thus an object of the invention to provide an implantable article, such as an implantable bone prosthesis having a strong body and long-wearing surface characteristics.
It is also an object of the invention to provide an implantable bone prosthesis that integrally combines hard exterior surfaces with structural strength and rigidity.
Another object of the invention is to provide casting techniques that enable the manufacture of implantable bone prostheses of hybrid structure having two or more materials of dissimilar properties.
Another object of the invention is to manufacture bone prostheses having two or more dissimilar materials which are interconnected in a high temperature process into a unitary body having reduced internal strain yet great structural strength and surface hardness.
The manner in which the invention attains these and other objects will be understood from the description that follows.